This invention relates to a mobile unit information sharing system wherein, among multiple mobile units (mobile stations) and a base station supervising the movements of the mobile units, position information of the mobile units is shared among the stations using a common wireless frequency.
Mobile unit communication systems of related art wherein voice signals are transmitted and received using a common wireless frequency include individual mobile unit information management systems in which information on individual mobile units is managed by a central station. An example of this appears in Japanese Patent Laid-open Publication JP-A-2001-43500 (Patent Document 1).
And in information sharing systems using wireless communications of related art, in communicating information such as the positions and activity states of multiple mobile units, and site images and so on, to avoid communication collisions and information bias, information is shared by being collected and managed by an information sharing server at a headquarters and distributed from there to the mobile units. Examples of this appear in Japanese Patent Laid-open Publication JP-A-2001-18837 and JP-A-2001-289642 (Patent Documents 2, 3).
Also, data transfer systems are known which perform time-sharing transmission control wherein multiple terminal stations are provided with a common reference time base and by a predetermined time within the period of the reference time base being allocated to each terminal station as a unique transmission timing-the overlapping of transmission timings in communications among the multiple stations is avoided without communications control being received from a base station. An example of this appears in Japanese Patent Laid-open Publication JP-A-08-130774 (Patent Document 4).
With a mobile unit communication system that transmits and receives voice signals using a common wireless frequency of related art such as that of JP-A-2001-43500 (Patent Document 1), although individual information on the mobile units can be managed at the central station, there has been the problem that on the mobile unit side it is not possible to check position information of other stations on a display and information of the mobile units cannot be shared among the stations.
In information sharing systems using wireless communications of related art such as those of JP-A-2001-188837 and JP-A-2001-289642 (Patent Documents 2, 3), because as mentioned above an information sharing server manages the shared information and controls the communications, there has been the problem that when this server fails to function normally, information sharing not only between the headquarters and the mobile stations but also among the mobile stations becomes difficult.
And when as in JP-A-08-130774 (Patent Document 4) and the like time-sharing transmission control is carried out wherein multiple terminal stations are provided with a common reference time base and by a predetermined time within the period of the reference time base being allocated to each terminal station as a unique transmission timing the overlapping of transmission timings (communication collisions) in communications among the multiple stations is avoided without communications control being received from a base station, because a predetermined time within the period of the reference time base is allocated to each terminal station, it is not possible for the transmission period to be changed as necessary. And, when transmitting high-volume data such as images, even if there are empty time slots that no other station is using, there has been the problem that because data can only be transmitted within the transmission periods allocated to the stations, the communications slots cannot be utilized effectively, and information cannot be shared efficiently.